1. Field of the Invention
The present invention relates to signal routing techniques to reduce transmission line reflections in electronic systems.
2. Description of the Related Art
The personal computer industry is evolving quickly as new innovations occur. The driving force behind this innovation has been the increasing demand for faster and more powerful computers. As technology advances, the density of electronic packaging of electronic components are increasing, and these components are being operated at ever increasing frequencies.
A typical computer or electronic system includes electrical components mounted on one or more printed circuit boards (PCBs). The electrical components communicate with each other through electrical signals provided on electrical conductors or traces routed on the PCB connected to the terminals of the electronic components. As the speed of the electronic system is increased, the routed conductors or traces on the PCBs carrying these signals begin to behave like transmission lines. The rising or falling edges of the higher frequency signals on the traces cause transmission line reflections. At lower frequencies, the reflections are insignificant since the transmission wavelengths of the signal transitions are relatively long compared to the length of the trace, thereby allowing reflections to settle relatively quickly. However, as the signal frequency increases, the signal wavelength approaches the same length as the traces on the PCB, and transmission line reflections become a significant factor causing noise on the signals of the electronic system.
The noise is primarily due to the signals being reflected back and forth on the trace, wherein these reflections can attenuate or reinforce the signal depending upon the phase relationships of the reflections. Signal noise appears in the form of ringing or bouncing, as well as undershoot and overshoot of the signals if the transmission line reflections are not properly compensated. Significant levels of signal noise results in false signal levels on the routed traces appearing at the inputs of the electrical components, eventually leading to failure of the system.
Several routing techniques have been developed to reduce the effects of transmission line reflections in electronic systems, such as daisy chaining between the source and the loads, avoiding 90 degree angles of the traces, and reducing the number of vias between PCB board layers. These techniques are still desirable and should be used when practicing the routing technique of the present invention. Other techniques, such as routing from the source to each load in series and providing an ac or dc terminator at or near the last load, have proved inadequate as the frequency is increased beyond a certain point. An ac terminator usually comprises a resistor and a capacitor coupled in series between the last load of a trace and ground, wherein the value of the resistor and capacitor is chosen to match the impedance of the signal trace together with the loads appearing on the trace. The ac terminator at high frequencies, however, tends to absorb some power, which is undesirable if the source is providing only enough power to drive the loads. Also, having to provide multiple terminators throughout the system substantially increases cost. As the speed of electronic systems increases, new routing techniques are becoming more and more important in order to improve the signal integrity of the systems without adding substantial cost.
The present invention relates to a new signal routing technique for routing traces between signal components in electronic systems, providing reduced transmission line reflections especially at higher frequencies.